1. Field of the Invention
The present invention relates to a carbon substrate and a process for production thereof, said carbon substrate being suitable for use as a substrate of magnetic disk, recording head, optical lens mold, optical reflector, and photosensitive drum.
2. Description of the Prior Art
Carbon substrates of this kind have been produced by filling a phenolic resin into a mold, forming a molded article of desired shape by hot pressing, and heat-treating the molded article in an inert atmosphere at a high temperature (say, 1200.degree. C.) for carbonization. The carbon substrate produced in this manner has a vitreous structure and hence it is light in weight and superior in surface flatness, surface roughness, and impact resistance to the conventional aluminum, glass, or ceramic substrate for magnetic recording medium. The carbon substrate for magnetic recording medium permits the recording density to be increased more, which in turn makes it possible to reduce the size of magnetic recording medium.
The above-mentioned carbon substrate has defects due to impurities (or ash) originating from the raw material resin. There has been proposed a method of eliminating such defects by limiting the ash content in the raw material resin to 5 ppm or less. (Japanese Patent Laid-open No. 260605/1992). However, it has turned out that the carbon substrate made from a resin containing less than 5 ppm of ash still has such a large number of graphitic defects that it does not have the quality required of magnetic recording medium. In other words, merely reducing the ash content in the raw material resin does not provide the carbon substrate having a minimum of defects, satisfactory for magnetic recording medium.
The carbon substrate may have defects due to voids. Voids occur in the molded article when the resin is cured or the molded article is carbonized. Voids results from gases formed by the thermal decomposition of the resin and also from moisture contained in the resin itself.
There was proposed a method of producing a void-free substrate of glass-like carbon. (Japanese Patent Publication No. 321/1989) The method employs as the raw material a thermosetting resin which has a viscosity of 300-8000 cP (at 25.degree. C.) in its initial condensation state before curing and can contain more than 20 wt % water. The thermosetting resin permits water (formed by curing) and gases (formed by decomposition) to be uniformly dispersed into itself during its curing. This prevents the occurrence of voids.
There was also proposed a method of producing glass-like carbon of compact structure with a minimum of voids. (Japanese Patent Publication No. 55122/1992). This method employs a strong acid (catalyst) as the hardener for the initial condensate of the thermosetting resin as the raw material. This hardener causes the polymerization reaction to proceed slowly and uniformly, thereby making it possible to effectively remove condensation water and decomposition gas.
Moreover, there was proposed a method of preventing the occurrence of voids. (Japanese Patent Publication No. 320210/1989). This method consists of curing the resin and performing postcuring and carbonizing at a controlled: pressure, heating rate, and temperature, such that the rate of water formation is lower than the rate of water diffusion in the resin.
The method disclosed in JP No. 321/1989 has a disadvantage of requiring the troublesome procedure to determine the capacity of the resin to hold water in the initial condensation stage. In addition, it does not clearly indicate the difference between the water holding capacity of the resin in the initial condensation state and the water holding capacity of the resin in the stage of curing and carbonization (in which water is actually formed).
The method disclosed in JP Nos, 321/1989 and 55122/1992 is intended to cause the condensation water and decomposition gas (which occur when the initial condensate as the raw material is cured) to be uniformly dispersed in the matrix resin, thereby preventing the occurrence of voids. Therefore, it has a disadvantage of requiring a long time (say, tens of hours, or even longer than 100 hours) for curing. This leads to a low productivity. It has another disadvantage which results from using an initial condensate as the raw material. That is, the initial condensate gives off a large amount of condensation water and decomposition gas before curing is completed. Their complete elimination is difficult to achieve; some of them (in the form of bubble) remain in the material, giving rise to voids.
The method disclosed in JP No. 320210/1989 has a disadvantage of requiring an extremely troublesome procedure to cure the resin and to perform the postcuring and carbonization. This leads to a high production cost.
In addition, a molded article of conventional glass-like carbon contains a large number of small masses (tens of micrometers in size) which differ in structure and quality from the surrounding. When polished, its surface has pits due to such small masses. Although these pits are only about 100 nm deep, they are not negligible in the application area (such as magnetic disk substrate) where a high degree of flatness and smoothness is necessary for high recording density without errors. Thus there is a demand for a high-performance molded article of glass-like carbon free from such defects.
As mentioned above, the conventional technology was unable to produce a molded article of glass-like carbon having a high degree of surface smoothness. In addition, it needed not only rigid control to cure the initial condensate of the raw material but also a long time for curing. This leads to low productivity and high production cost.